1. Field of the Invention
This invention relates generally to spin valve magnetic transducers for reading information signals from a magnetic medium and, in particular, to a spin valve sensor with enhanced GMR effect and improved thermal stability, and to magnetic storage systems which incorporate such sensors.
2. Description of Related Art
Computers often include auxiliary memory storage devices having media on which data can be written and from which data can be read for later use. A direct access storage device (disk drive) incorporating rotating magnetic disks is commonly used for storing data in magnetic form on the disk surfaces. Data is recorded on concentric, radially spaced tracks on the disk surfaces. Magnetic heads including read sensors are then used to read data from the tracks on the disk surfaces.
In high capacity disk drives, magnetoresistive (MR) read sensors, commonly referred to as MR heads, are the prevailing read sensors because of their capability to read data from a surface of a disk at greater track and linear densities than thin film inductive heads. An MR sensor detects a magnetic field through the change in the resistance of its MR sensing layer (also referred to as an "MR element") as a function of the strength and direction of the magnetic flux being sensed by the MR layer.
The conventional MR sensor operates on the basis of the anisotropic magnetoresistive (AMR) effect in which an MR element resistance varies as the square of the cosine of the angle between the magnetization in the MR element and the direction of sense current flow through the MR element. Recorded data can be read from a magnetic medium because the external magnetic field from the recorded magnetic medium (the signal field) causes a change in the direction of magnetization in the MR element, which in turn causes a change in resistance of the MR element and a corresponding change in the sensed current or voltage.
Another type of MR sensor is the giant magnetoresistance (GMR) sensor manifesting the GMR effect. In GMR sensors, the resistance of the MR sensing layer varies as a function of the spin-dependent transmission of the conduction electrons between magnetic layers separated by a non-magnetic layer (spacer) and the accompanying spin-dependent scattering which takes place at the interface of the magnetic and non-magnetic layers and within the magnetic layers.
GMR sensors using only two layers of ferromagnetic material (e.g., Ni--Fe) separated by a layer of non-magnetic material (e.g., copper) are generally referred to as spin valve (SV) sensors manifesting the SV (GMR) effect. In an SV sensor, one of the ferromagnetic layers, referred to as the pinned layer, has its magnetization typically pinned by exchange coupling with an antiferromagnetic (e.g., NiO or Fe--Mn) layer. The pinning field generated by the antiferromagnetic layer should be greater than demagnetizing fields (about 200 Oe) at the operating temperature of the SV sensor (about 120 C) to ensure that the magnetization direction of the pinned layer remains fixed during the application of external fields (e.g., fields from bits recorded on the disk). The magnetization of the other ferromagnetic layer, referred to as the free layer, however, is not fixed and is free to rotate in response to the field from the recorded magnetic medium (the signal field). In SV sensors, the SV effect varies as the cosine of the angle between the magnetization of the pinned layer and the magnetization of the free layer. Recorded data can be read from a magnetic medium because the external magnetic field from the recorded magnetic medium (the signal field) causes a change in direction of magnetization in the free layer, which in turn causes a change in resistance of the SV sensor and a corresponding change in the sensed current or voltage. IBM's U.S. Pat. No. 5,206,590 granted to Dieny et al., incorporated herein by reference, discloses a GMR sensor operating on the basis of the SV effect.
FIG. 1 shows a prior art SV sensor 100 comprising end regions 104 and 106 separated by a central region 102. A free layer (free ferromagnetic layer) 110 is separated from a pinned layer (pinned ferromagnetic layer) 120 by a non-magnetic, electrically-conducting spacer layer 115. The magnetization of the pinned layer 120 is fixed by an antiferromagnetic (AFM) layer 125. Free layer 110, spacer layer 115, pinned layer 120 and the AFM layer 125 are all formed in the central region 102. Hard bias layers 130 and 135 formed in the end regions 104 and 106, respectively, provide longitudinal bias for the free layer 110. Leads 140 and 145 formed over hard bias layers 130 and 135, respectively, provide electrical connections for the flow of the sensing current I.sub.S from a current source 160 to the MR sensor 100. Sensing means 170, typically includes a recording channel such as a partial-response maximum-likelihood (PRML) channel, connected to leads 140 and 145 sense the change in the resistance due to changes induced in the free layer 110 by the external magnetic field (e.g., field generated by a data bit stored on a disk).
As mentioned above, the magnetization of the pinned layer 120 in the prior art SV sensor 100 is generally fixed through exchange coupling with AFM layer 125 of antiferromagnetic material such as Fe--Mn or NiO. However, both Fe--Mn and NiO have rather low blocking temperatures (blocking temperature is the temperature at which the pinning field for a given material reaches zero Oe) which make their use as an AFM layer in an SV sensor difficult and undesirable.
A desirable alternate AFM material is Ni--Mn which has much higher corrosion resistance than Fe--Mn, very large exchange pinning at room temperature, and much higher blocking temperature than either Fe--Mn or NiO. High blocking temperature is essential for SV sensor reliability since SV sensor operating temperatures can exceed 120 C in some applications.
Referring to FIG. 2, there is shown the change in the unidirectional anisotropy field (H.sub.UA) or pinning field versus temperature for 50 .ANG. thick Ni--Fe pinned layers using Fe--Mn, NiO and Ni--Mn as the pinning layers. Fe--Mn has a blocking temperature of about 180 C (curve 210) and NiO has a blocking temperature of about 220 C (curve 220). Considering that a typical SV sensor used in a magnetic recording disk drive should be able to operate reliably at a constant temperature of about 120 C with a pinning field of at least 200 Oe, it can readily be seen that Fe--Mn substantially loses it ability to pin the pinned layer at about 120 C (pinning field dropping to about 150 Oe) and NiO can only marginally provide adequate pinning at about 120 C (pinning field dropping to about 170 Oe). It should be noted that once the pinning effect is lost, the SV sensor loses its SV effect either totally or partially, rendering the SV sensor useless. In contrast, it can be seen in FIG. 2 that Ni--Mn with a blocking temperature of beyond 450 C (curve 230) easily meets the pinning field requirements at the 120 C operating temperature of typical SV sensors.
However, the problem with using Ni--Mn AFM for the pinning layer is the requirement for a high temperature (equal or greater than 240 C) annealing step after the deposition of the SV sensor layers (post-annealing) to achieve the desired exchange coupling between the Ni--Mn pinning layer and the Ni--Fe pinned layer in order to achieve proper SV sensor operation. Unfortunately, annealing at such high temperature (equal or greater than 240 C) substantially degrades the GMR coefficient of the SV sensor. This irreversible degradation of the SV sensor is believed to be caused by interdiffusion at layer interfaces. Stability against this interdiffusion is a prerequisite for the use of Ni--Mn as the AFM layer in an SV sensor because the SV sensor must survive the severe heat treatment required to anneal the Ni--Mn. In addition, the magnetostriction of the free layer increases sharply after annealing. This increase in the magnetostriction is believed to be caused by intermixing at the interface between the seed layer and the free layer.
Therefore there is a need for an SV sensor using a Ni--Mn AFM pinning layer that can withstand the annealing step required to achieve the desired exchange coupling without the undesirable degradation of the SV effect and without problems in control of magnetostriction of the free layer.